The present invention relates generally to an apparatus for tensile testing flat or plate-type ceramic specimens to determine the strength properties of various ceramics, and more particularly to such apparatus incorporating a self-aligning pull rod system utilized for gripping such ceramic specimens during the tensile testing thereof so that no or essentially no bending stresses will be applied to the specimen during the testing thereof.
The determination of the mechanical properties of ceramic materials is becoming of increasing importance with the advert of recently developed ceramics which have been found to be useful as structural materials in many engineering applications such as fabrication of heat engines and gas turbines. Accuracy in the testing of the ceramic materials to determine the strength, especially the tensile strength, thereof is of considerable importance regarding the use of the ceramic materials in most engineering applications. In such tensile testing of ceramics, it is particularly critical to the accuracy of the test to apply the tensile loading on the ceramic specimen without introducing deleterious bending stresses in the specimen since non-uniformly applied loadings on the specimen can bias test results due to the random nature of process defect distribution pre-existing in the specimen coupled with the high sensitivity of fracture initiation characteristics inherent to brittle ceramic materials.
A recent development in tensile testing ceramic specimens is described in U.S. Pat. No. 4,686,860 which issued Aug. 18, 1987 in the name of Kenneth C. Liu, the inventor in the instant application. In this patent, a self-aligning multiple hydraulic piston assembly and specimen gripping arrangement is provided for applying tensile loadings on ceramic specimens of cylindrical configurations while transmitting little or no bending stresses to the ceramic specimens. However, while this patented development provides highly accurate tensile testing of cylindrical specimens, the tensile testing of flat or plate-type ceramic specimens, especially ceramic specimens having the so-called "dogbone" shape as defined by generally T-shaped end sections, without introducing undesirable bending stresses in test conditions has not yet been satisfactorily addressed. One of the principal problems encountered during the tensile testing of the flat ceramic specimens is that the tolerances used in the formation of the specimen by machining or any other technique are highly critical in order to assure that the no or essentially no bending stresses will be applied to the tensile specimen during the application of a tensile load thereon.
For example, a previous effort used for tensile testing dogbone-shaped, plate-type ceramic specimens involved the placement of clevis pins at each end of the specimen for assuring that the tensile loading applied through the clevis pins is directed along the longitudinal center of the specimen. This clevis pin fixture requires that holes be machined at precise locations in the opposite ends of the specimen for receiving the clevis pins which are used to attach the specimen to a tensile load applying mechanism. The provision of these holes in the ceramic specimen was found to be difficult to achieve, especially in a reproducible manner, for accurately aligning the specimen with respect to the direction of applied force.
Another effort previously used for tensile testing dogbone-shaped plate-type tensile specimens is described in a technical article entitled "Towards Routine Tensile Testing" by Tatsuki, OHJI, Government Research Institution, Nagoya, Japan, Int. J. High Technology Ceramics, Vol. No. 4, 1988. In this technical article the specimen-gripping pull rod mechanism utilizes pins which bear against the shoulders on the T-shaped ends of each specimen for holding the specimen in the pull rod. Each pull rod is provided with universal joints for load distribution purposes. The tensile specimens used in this specimen-gripping mechanism must be machined to precise and reproducible tolerances so as to be accurately tested without encountering deleterious bending stresses. However, such precise machining is of considerable expense and difficult to achieve, especially in a reproducible manner.
A further effort for tensile testing plate-type ceramic specimens is described in another technical article entitled, "Tensile Testing of Ceramics" by S. P. Sehadri et al, Journal of American Ceramic Society, Volume 70, Oct. 1987, pp. C-242 to C-244. In this article the tensile specimens are provided with wedge-shaped ends each of which is confined between two pin-mounted, flat-sided grip blocks which are rotatable about the pins by the wedge-shaped specimen to align the specimen in a plane parallel with the pull rod during the application of the tensile loading. While this fixture apparently obviates the requirement for precise machining of the specimens and minimizes bending stresses on the specimen during testing, the fixture requires the use of wedge-shaped specimens which require a configuration with a strict symmetry with respect to the loading axis rather than the more commonly used and easier formed dogbone-shaped plate-type specimens.